Traveling wave tube



May 13, 1953 4 E. c. DENCH 2,834,915

TRAVELING WAVE. TUBE.

Original Filed Oct. 30, 1953 2 Sheets-Sheet 1 //v VEN TOI? EDWARD C.DEA/CH y 13, 1958 E. c. DENCH 2,834,915

IRAVELING WAVE TUBE Original Filed Oct. 50, 1953 2 Sheets-Sheet 2 FIG./2

. INVENTOI? EDWARD C. DENCH ATTORNEY United States Patent Office2,834,915 Patented May 13, 1958 TRAVELING WAVE TUBE Edward C. Dench, Needham, Mass, assignor to Raytheon Manufacturing Company, Waltham,Mass., at corporation of Delaware Original application October 30, 1953,Serial No. 389,415. Divided and this application March 4, 1955, SerialNo. 492,073

4 Claims. (Cl. 315-393) This application is a division of application,Serial No. 389,415, filed October 30, 1953, by Edward C. Dench.

This invention relates to improved techniques of constructing travelingwave electron discharge devices and the anode periodic structures usedtherein.

If a traveling wave tube is to function properly, the periodic anodedelay line used therein must be mechanically accurate so that the properphase relation between the electron beam and the traveling wave energyis maintained. As the operating frequency increases, the physical sizeof the elements of the periodic network, such as the fingers of aninterdigital line, tends to decrease so that still greater accuracy ofconstruction of the anode delay line must be attained at microwavefrequency.

When the power requirements of the traveling wave tube are relativelysmall, a high order of precision in the construction of anode delaylines may be obtained by resorting to a printed circuit technique.Although several printed circuit techniques are available, perhaps themost accurate is the photoengraving process. An electri-Cally-insulating base is plated with a thin film of electri-Cally-conductive material, such as copper. A carefully constructedlayout is made of the delay line and a picture of the line is taken withan appropriate camera. A light sensitive enamel which is also resistantto acids is applied to the metallized surface of the insulating base orsupport. The negative obtained by the aforesaid photographic means ispositioned over the sensitized surface and light from a source ofvisible radiation is directed thereupon. The unexposed portion of theenameled surface is washed away and the metallized layer not coated byenamel is etched. After removing the remaining enamel resist, themetallized surface remaining on the insulating base forms an exactreplica of the desired anode delay line. Once a negative has been madeof the layout,

any number of anode delay lines may be manufactured,

all of the same high degree of accuracy.

Alternatively it is possible to apply an etching ink to the metallizedlayer through the apertures of a circuitdefining stencil to coat the inkwith an acid resistant powder which is fused under heat, and to etch andclean as before.

Other methods of printing which may be used for forming the anode delaylines are painting or spraying a conductive film directly on aninsulating base and then firing, an electrolytic process, or vacuumsputtering or evaporation techniques. 7

Another modification of a periodic anode delay line is a laminatedstructure. This is particularly adapted for use in an interdigitalperiodic delay line, such as described and illustrated in an'applicationfor U. S. Letters Patent, SerialNo. 382,025, of E. C. Dench, filedSeptember 24, 1953. These interdigital lines have been found verysatisfactory for use in low power traveling wave amplifiers andoscillators and are characterized by good dispersion characteristics anda reasonably high impedance. Each half of the anode may be punched orstamped sepa i2. rately in the form of a comb and the two laminatedportions positioned together and fastened to an appropriate base. Thelaminated periodic delay line, like that obtained by the printingtechniques referred to above, is highly accurate, in addition to beingrelatively inexpensive to produce and readily reproducible.

Each of the techniques above described may be used in constructing othertypes of anode delay lines. For example, the photoengraving methodpreviously described may be used to produce a helical conductor forhelix-type traveling wave tubes.

The foregoing and other features of this invention will become moreapparent from the detailed description of certain specific embodimentswhich follow. The description refers to the accompanying drawingswherein:

Fig. 1 is an isometric view showing a traveling wave tube incorporatingan interdigital anode delay line obtained by printed circuit techniques;

Fig. 2 is a fragmentary cross-sectional view of the tube of Fig. 1showing the anode delay structure;

Figs. 3 to 5 illustrate certain steps in the manufacture of the anodestructure of Fig. 1;

Figure 6 illustrates an alternative method of manufacturing theaforesaid structure;

Figs. 7 to 10 are illustrative of a method of manufacturing acylindrical anode delay structure by a printed circuit technique;

Fig. 11 is a view of a tubular member such as shown in Fig. 8 modifiedfor producing a helix rather than an interdigital delay line; a

Fig. 12 is a .view showing a typical pair of laminations for use in aninterdigital anode structure;

Fig. 13 is a plan view of the laminations of Fig. 12 as assembled toform an interdigital anode delay line; and

Fig. 14 is a cross-sectional view of the interdigital anode of Fig. 13.

. Referring to Fig. 1, a traveling wave oscillator tube 20 is shownincorporating a linear periodic anode structure 22. This anode delayline, which is formed in a manner to be described later, is carried byan electrically insulating base plate 24 which, in turn, is securelyattached by any appropriate means to a continuouselectrically-conductive backing member 25. Backing member 25 forms oneof the walls of an evacuated envelope which further includes anoppositely disposed wall 26, side walls 27 and 28 and a pair of endwalls, one of which, namely wall 30, is shown in Fig. 1.

The interdigital anode network 22, like that described in the aforesaidcopending application, comprises a pair of electrically-conductiveinterleaved assemblies or combs 31 and 32, each including a longitudinalportion and a plurality of spaced fingers which connect to saidlongitudinal portion and extend transverselyto the latter and almost tothe opposite assembly.

Attenuation may be introduced into the interdigital anode network 22 bymeans of a coating of lossy material, such as iron, which may be platedon the fingers over the region of the network in which attenuation isdesired.

The inner conductor 34 of a coaxial coupling device 35 extends throughan aperture 36 in wall 25 and base plate 2-4 and is attached, as bysoldering, to an end finger of the interdigital line, as clearly shownin Fig. 1.

Although the device as shown in Fig. 1 has only one energy couplingmeans, and therefore is adapted to operate as the traveling waveoscillator, it should be understood that the invention is not solimited, but may be applied equally well to a traveling wave amplifier,provided, of course, that the electron beam is synchronized with theproper space harmonic of the electro-magnetic field.

The traveling wave tube 20 further includes a cathode structure 38having an electron emissive surface 39; this cathode is positionedadjacent the output end of the structure. Cathode 38 is supported by ahollow supporting cylinder 40 extending through an aperture in wall 26of the tube envelope. A central conductor 41 is located inside cylinder40 of a coaxial support assembly 47 and is connected to one end of aheater coil, also not shown, positioned in thermal proximity to thecathode emissive surface. Cylinder 40 is insulatedly supported withrespect to wall 26 by means of a metallic member 44 sealed, in turn, toinsulating seals 45. The latter are connected to anelectrically-conductive cylinder 46 which surrounds cylinder 44) and is,in turn, attached to recesses in wall 26 surrounding the aperturesthrough which cylinder 40 passes. The construction of this cathode isset forth in considerable detail in an application for U. S. LettersPatent, Serial No. 255,499 of E. C. Dench, filed November 8, 1951, nowPatent No. 2,809,328, dated October 8, 1957.

An auxiliary electrode or sole 42 is disposed substantially parallel tothe anode network and spaced therefrom. Sole 42 is a U-shaped memberwhich is supported with respect to the tube envelope by means of a pairof supporting rods 43 rigidly attached to the sole and forming part of acoaxial support assembly 47 similar in construction to that of assembly47 used for support of the cathode.

Positioned at the opposite end of the sole from the cathode and insubstantial alignment with the sole is a collector electrode 48 rigidlysupported by means of a rod 49 extending through an aperture in wall 26and spaced from said wall. Rod 49 is supported relative to wall 26 bymeans of a terminal support assembly 50 similar to that used for sole42. In some applications, the collector electrode 48 may be omitted.

An electric field may be established between the anode and sole byconnecting a source 52 of voltage therebetween. The necessary electricalconnection between anode 22 and wall may be made by means ofelectrically-conductive screws or rivets (not shown) used for attachingthe anode assembly, including base 24, to the tube envelope. The cathode38 is negative with respect to anode 22 and may or may not be at thesame potential as sole 42.

A transverse magnetic field is produced in the space between theperiodic anode structure and the sole in a direction perpendicular tothe electric field. The pole pieces 54 and 55 of the magnetic fieldproducing means are shown in Fig. 2. By proper adjustment of themagnetic field, the electrons emitted from the cathode 38 will bedirected along a path adjacent the fingers of the periodic structure 22.Interaction of the electron beam.

with a backward wave traversing the periodic anode structure will resultin the generation of oscillations in the tube. See the prior comments asto the adaptability of the periodic anode structure in a traveling waveamplifier.

One method of producting the interdigital anode delay line is shown inFigs. 3 to 5. For purposes of clarity the thickness of certain layers ofthe material and circuit defining objects is exaggerated.

A layout of the particular interdigital anode desired is made, as, forexample, by India ink on white drawing paper. A picture is taken of thislayout by means of a camera having a reasonably high quality lens. Aprinting negative 60 is then produced by the usual photographictechniques so that the light portions thereof correspond inconfiguration to that of the interdigital line, while the dark portionscorrespond to the portion of the original layout not covered by drawingink.

The insulating base plate 62, which may be of ceramic, glass or otheracid resistant electrically-insulating materials, is coated, as by anywell-known plating process, with a thin layer or film 63 of metal, suchas copper.

The copper-clad plate is next coated with a light-sensitive enamel 64which is impervious to acid, such as a dichromate solution. After theenamel has dried sufficiently, the negative 60 is placed over thesensitized base plate 62-64 and the masked plate is exposed to a highintensity source 65 of light for a proper time, dependent upon the typeof enamel and the type of light source used, as shown in Fig. 3.

The light passing through the transparent regions of negative 60 causesthe portion of the enamel layer 64 corresponding to the transparentregions to harden. After removing the negative, the unhardened portionof the enamel surface is washed away by means of an appropriate enamelsolvent, such as alcohol or water, leaving a structure as shown in Fig.4. The plate is next exposed to heat until the enameled surface stillextant is completely dried and hardened. This surface 64 now serves as aresist which is impervious to the action of acids.

The plate is next etched in an acid solution which is chosen inaccordance with the material of which the anode delay line 22 is to bemade. If the metallized layer 63 is made of copper, a solution of ferricchloride may be used for etching purposes. As soon as the areas of thecopper layer 63 on base plate 62 not protected by the enamel resist 64are completely removed by the chemical action of the etching solution,the plate is rinsed, thereby leaving the enamel covered interdigitalanode line shown in Fig. 5. Finally the enamel resist 64 is removed byan appropriate enamel remover, such as a caustic potash solution. Thecopper-clad base plate bearing a desired circuit design is now ready formounting by any desired means to the wall 25 of traveling wave tube 20.

In Fig. 6 an alternate approach for producing the interdigital line ofFig. 1 is shown. An electrically-insulating base plate 62 is coated witha thin layer 63 of copper, silver or other conductive material, asbefore. A stencil 66 having apertures 67 therein defining the desiredanode line is placed over the metal-clad base plate 62, 63 and anetching ink applied, as by a brush 68, to the surface of the metal-cladplate through the stencil apertures. The inked image on the plate isprotected from the action of acid by means of an acid resisting powder,such as com'minuted resin or shellac, which, when poured over the greasyetching ink, adheres thereto. The plate is next inserted in an etchingbath suited to the particular metal of which the metallic layer 63 isconstituted. For example, a copper layer may be etched by the ferricchloride solution previously referred to or by an aqueous solution ofnitric acid. Finally the resist (combination of ink and resin) isremoved from the plate by means of a strong solution of caustic potash.

It is, of course, possible to form the interdigital line 22 by applyingconductive paint, such as a lacquer containing a relatively largeproportion of metal powder, directly to the base plate 62 through astencil 66 of Fig. 6. This paint may be applied either by brushing orspraying.

In Figs. 7 to 10, a means for producing a cylindrical anode interdigitaldelay line suitable for use in a cylindrical traveling wave tube isshown. An electrically-insulating tube 70 made of glass, quartz orceramic is coated on the inside with a layer 63 of copper or otherelectrica'lly-conductive material approximately .002 inch thick. Thethickness, however, may vary with the material being used and the powerrequirements of the traveling wave tube. A photosensitive layer 64, suchas already described, is then applied over the metallic surface 63, asshown in Fig. 7. An optically transparent plastic tube 74, whose outsidediameter is slightly less than the inner diameter of tube 70, whencoated, is painted with a material which is opaque to light, as shown inFig. 8.

The pattern on tube 74 may also be formed by wrapping a photographicnegative around the outside of the tube. This tube which corresponds tothe negative 60 of Fig. 3 is preferably, though not necessarily, splitso that it may be resiliently adapted to fit snugly Within the tube 70.*In preparing tube 74 it is necessary to leave the tube uncoated overthe areas corresponding to the various fingers of the desiredinterdigital anode line.

Tube 74, of course, may be made of any resilient material which istransparent to light. As shown in Fig. 7,

the partially transparent tube 74 resulting from the application of thecoating is shown in Fig. 8. Tube 74, having alternate regions which aretransparent and opaque to light, is placed within tube 70 of Fig. 7 anda light source, such as a tubular are lamp 75, is next inserted withintube 74. After the necessary exposure has been obtained, the lightsource 75 and transparent tube 74 are removed and the unh-ardenedportion-of the photosensitive layer 64 is washed away. After etching andremoval of the hardened portion of the photosensitive layer, a periodicstructure 22 having arcuate fingers 77 connected to and extending from alongitudinally disposed base element 76 is obtained, as shown in Figs. 9and 10. A coaxial input and output coupling means 78 and 79,respectively, are shown in Fig. 9. An electron beam produced by astandard type electron gun is directed longitudinally adjacent thevarious fingers 77 of the line as indicated by the arrow in Fig. 9. Theinner conductors 80 and 81 of respective coaxial means 78 and 79 may beattached to the end fingers of the line, as by soldering. The use of twoenergy coupling means at opposite ends of the anode delay linepresupposes operation of the associated traveling wave tube as anamplifier. If a traveling wave oscillator is desired, it is obvious thatonly a single energy coupling means would be used.

The construction of the printed anode line and the connection of theenergy coupling means to the line is more clearly shown in Fig. 10 inwhich the line has been removed from the inner surface of the tube 70.

In Fig. 11, plastic tube 74 is coated with a helical layer of material83 which is opaque to light. The tube 74 is positioned within tube 70and the process previously described in connection with Figs. 7 to 9 isrepeated. In i this manner a helix is formed on the inner surface oftube 70 instead of an interdigital line. Although Figs. 7 to 11 havedisclosed a helix and inter-digital line, it is possible to form anodedelay lines of other configurations by means of the invention.

In Figs. 12 to 14, an additional approach to the problem of constructinginterdigital anode delay lines for traveling wave tubes is illustrated.In Fig. 12 the two opposing halves 85 and 86 of the interdigital line 22are stamped or otherwise formed from sheets of electricallyconductivematerial, such as copper. These laminations may containcircularapertures 87 for mounting purposes. The thickness of the sheetswill be determined, to a large extent, on the power handlingrequirements of the traveling wave tube incorporating said anodestructure. For example, for relatively low power tubes, the laminations85 and 86 may be made from metal foil. When higher power tubes aredesired, several identical sheet metal laminations may be stackedtogether as shown in Fig. 14. The selective arrangement of thelaminations 85 and 86 of Fig. 12 is shown in Fig. 13. The laminationsmay be fastened directly to the wall 25 of traveling wave tube 20, as byrivets 90.

In Fig. 14, the stacked laminations 85 and 86 are shown fastened to base88 as by screws 89 inserted through aligned apertures in the laminationsand base. Base 88 may be fiat, or U-shaped, as shown in Fig. 14.

It should be understood that this invention is not limited to theparticular details of construction, materials and processes described,as many equivalents will suggest themselves to those skilled in the art.For example, the traveling wave tubes need not be linear, as shown inthe drawings, but may have other configurations such as circular orarcuate. In addition, the modifications shown 6 in the drawings may beapplicable either to traveling wave amplifiers or oscillators.Therefore, it is desired that the appended claims be given a broadinterpretation commensurate with the scope of the invention within theart.

What is claimed is:

1. A traveling wave tube comprising an evacuated envelope containing acontinuous slow wave energy propagating structure for producing along apath adjacent thereto fields of electromagnetic wave energy beingtransmitted, a source of electrons, means for directing a beam of saidelectrons along the interaction space between said structure and saidelectrode and in energy interacting relation with said wave fields, saidmeans for directing including means for producing a magnetic fieldtransverse to said electron beam, 'an electrically-insulating basesupported by a portion of said envelope, said structure including anelectrically-conductive element printed upon said base,and energycoupling means connected through an aperture in said base to saidconductive element.

2. A traveling wave tube comprising an evacuated envelope containing acontinuous slow wave energy propagating structure for producing along apath adjacent thereto fields of electromagnetic wave energy beingtransmitted, a source of electrons, means for directing a beam of saidelectrons along the interaction space between said structure and saidelectrode and in energy interacting relation with said wave fields,.saidmeans for directing including means for producing a magnetic fieldtransverse to said electron beam, said structure including anelectrically-insulating base supported by a portion of saidenvel0pe,,said structure including a pair of interleavedelectrically-conductive elements printed upon said base, and energycoupling means connected through an aperture in said base to one of saidconductive elements.

3. A traveling wave tube comprising an evacuated envelope containing acontinuous slow wave energy propagating structure for producing along apath adjacent thereto fields of electromagnetic wave energy beingtransmitted, an electrode spaced from and substantially coextensive withsaid translation structure and maintained at a potential negative withrespect thereto, a source of elec trons, means for directing a beam ofsaid electrons along the interaction space between said structure andsaid electrode and in energy interacting relation with said Wave fields,an electrically-insulating base supported by a portion of said envelope,said structure including a pair of interleaved comb-shapedelectrically-conductive elements printed upon said base, and energycoupling means connected through said base to at least one of saidconductive elements.

4. A traveling wave electron discharge device comprising an electricallyconductive envelope, an electrically insulating member mounted on aportion of said envelope, a pair of thin interleaved electricallyconductive coatings attached to said insulating member, said coatingsforming an interdigital periodic slow wave propagating network, andenergy coupling means having a portion thereof connected to said networkthrough an aperture in said insulating member.

References Cited in the file of this patent UNITED STATES PATENTS2,611,101 Wallauschek Sept. 16, 1952 2,622,158 Ludi Dec. 16, 19522,654,842 Engelmann Oct. 6, 1953 2,768,322 Fletcher ,Oct. 23, 1956 OTHERREFERENCES Article entitled, Manufacture of Microstrip, EleotricalCommunications, vol. 29, pages 251-259, December 1952.

U. S. DEPARTMENT OF COMMERCE PATENT OFFICE CERTIFICATE OF CORRECTIONPatent No. 2,834,915 Edward C, Dench I May 13, 1958 It is herebycertified that error appears in the printed specification of the abovenumbered patent requiring correction and that the said Let ters Patentshould read as corrected below.

Column 3 line 59, for "producting" read a-producing-q column 6,; lines31 32, strike out "said structure includin 1 Signed and sealed this 1stday of July 1958,.

(SEAL) Attest: KARL H. LINE ROBERT C WATSON Attesting OfficerConmissioner of Patents

